Exemplary embodiments of the present invention relate to a mixing device for an exhaust system of a vehicle. The mixing device comprises an inlet pipe through which exhaust gas can enter a mixing chamber of the mixing device. The exhaust gas can leave the mixing chamber through an outlet pipe of the mixing device. The inlet pipe and the outlet pipe form an angle. The mixing chamber comprises an opening for an outlet of a dosage device, by means of which a reducing agent for exhaust gas aftertreatment is introducible into the mixing chamber.
PCT patent publication WO 2009/128 885 A1 describes a mixing device designed for the fine distribution of a reducing agent for exhaust gas aftertreatment, for example an urea-water solution. A mixing pipe is arranged within a mixing chamber formed by a pipe with a 90° angle. The mixing pipe has an opening adapted to receive a nozzle of a urea dosage device. A funnel-shaped end portion of the mixing pipe bears against a wall of the mixing chamber. The mixing pipe is provided with a multitude of large-area perforations on its circumference, which enable the exhaust gas to enter the mixing pipe. Other perforations in the funnel-shaped end portion of the mixing pipe lead to an exhaust gas flow along the circumference of an outlet pipe connected to the mixing chamber.
One disadvantage of this urea mixing device is that it leads to a considerable backpressure of the exhaust system. Additionally, in case of urea as the reducing agent, an enhanced danger for unwanted deposits may exist.
Extensive mixing of urea with the exhaust gas helps improve the conversion of nitrogen oxides in an SCR catalytic converter (SCR=selective catalytic reduction) arranged downstream of the mixing device in the exhaust system. In the SCR catalytic converter nitrogen oxides (NOx) contained in the exhaust gas react with ammoniac (NH3) released from the urea of the urea containing solution or such a reducing agent. Thus, nitrogen oxides are converted with the ammoniac to nitrogen and water in a selective catalytic reduction reaction. This leads to a decrease of the content of nitrogen oxides in the exhaust gas.
Exemplary embodiments of the present invention provide a mixing device that achieves an enhanced mixing of the reducing agent with the exhaust gas.
The mixing device according to the invention comprises a baffle element. This baffle element is arranged at a wall of the inlet pipe, and it is designed to induce a turbulence in the exhaust gas flow entering the mixing chamber. The baffle element guides the exhaust flow and introduces a turbulence or swirl in the flow field downstream of the baffle element. Therefore, turbulences are already induced at a location of the mixing device, where the reducing agent is injected into the mixing device and the mixing of reducing agent and exhaust gas takes place.
Such a mixing device significantly enhances the mixing of the reducing agent, in particular the mixing of urea with the exhaust gas. With urea as the reducing agent this leads to an improvement of the conversion efficiency regarding nitrogen oxides in an SCR catalytic converter arranged downstream of the mixing device in the exhaust system.
Due to the performance improvement of the mixing device a cost reduction in the operation of the mixing device can be achieved, as less reducing agent needs to be utilized. As the design of the mixing device with the baffle element arranged inside the inlet pipe is very simple, there are low maintenance costs and relatively few parts are incorporated in the mixing device.
Another advantage is the reduction in the chance of a deposit of the reducing agent inside the mixing chamber and downstream of the mixing chamber. This is also due to the open design of a volume of the mixing chamber close to the dosage device. In other words the mixing chamber is empty, i.e. devoid of any elements that would increase backpressure.
The turbulences introduced in the exhaust gas entering the mixing chamber very efficiently avoids the deposit of the reducing agent within the mixing chamber. The mixing device is therefore highly effective and a low cost exhaust gas aftertreatment component.
In an advantageous embodiment of the invention the baffle element is arc-shaped with an outer boundary contacting the wall of the inlet pipe and an inner boundary forming a section of a circular-arc. Such a baffle element has proven to be particularly effective in inducing turbulences in the exhaust gas flow and also guiding the flow direction of exhaust gas in an advantageous manner, while the backpressure is only slightly increased by the baffle element.
It has further proven to be advantageous if the baffle element is contacting the wall of the inlet pipe substantially along a quarter of an inner circumference of the inlet pipe. This leaves a large part of the inner circumference of the inlet pipe free of any flow guiding element, but nevertheless the baffle element is efficient in inducing turbulences.
A first section of the wall of the inlet pipe is located on the same side of the mixing device as the opening in the mixing chamber. A second section of the wall of the inlet pipe is located opposite the first section. Preferably at least a major part of the baffle element is arranged at a portion of the wall of the inlet pipe, which is located between this first section and the second section. In other words the baffle element is preferably arranged at other locations of the inlet pipe than the first section and the second section. However, the baffle element can be arranged in a quadrant of the inlet pipe adjoining an imaginary central plane going through the opening in the mixing chamber and dividing the inlet pipe into two halves. It has been found out that a particularly good distribution of reducing agent in the exhaust gas downstream of the mixing device can be achieved by such arrangements of the baffle element.
In a further advantageous development of the invention a width of the baffle element is less than 25% of a diameter of the inlet pipe. With such a relatively small height of the baffle element the mixing device can achieve particularly good backpressure values. This is particularly true if the width of the baffle element is about 10% of the diameter of the inlet pipe.
It is further advantageous if a portion of the mixing chamber, which is closer to the inlet pipe than to the outlet pipe, is substantially square-shaped. Such a shape of a main part or body part of the mixing chamber allows the exhaust flow to stay relatively long within the mixing chamber and to interact to a very high degree with the reducing agent. This further enhances the mixing of the reducing agent spray with the exhaust gas.
In a further advantageous development of the invention the mixing chamber comprises a bulge in a side wall facing the inlet pipe. This leads to an enhanced formation of turbulences or swirls in the mixing chamber itself and in the outlet pipe connected to the mixing chamber. The uniformity of mixing is largely improvable by this design of the mixing chamber. Also, the flow inertia and the characteristics of the exhaust gas-reducing agent interaction are utilized by this design of the mixing chamber. This bulge or pocket, which is located in a prolongation of the inlet pipe, has proven to be very effective in producing strong swirls in the exhaust gas flow.
During operation of the exhaust system the exhaust gas flow from the inlet pipe is guided by the flat side walls of the mixing chamber to the bulge or pocket at the bottom. The flow thus experiences one or more swirls in a volume close to the bulge before it turns towards the outlet pipe. The strong swirls induced by this design of the mixing chamber are effective even in the outlet pipe. This further enhances the mixing of the reducing agent with the exhaust gas.
A deposit of reducing agent within the bulge can be minimized, if according to a further advantageous development of the invention a portion of the bulge is bevelled, which is adjacent to a wall of the mixing chamber, in which the outlet opening of the dosage device is located.
By the sloped design of this portion of the bulge, which is adjacent to the wall of the mixing chamber with the opening, there is no flow recirculation in this region, which could otherwise lead to a deposit of the reducing agent within the mixing chamber. As such a deposit is critical for the performance of the aftertreatment device, bevelling this portion of the bulge improves the performance.
In order to minimize backpressure and to ease the flow from the inlet pipe into the mixing chamber and from the mixing chamber to the outlet pipe, it has further proven to be advantageous if the inlet pipe and the outlet pipe are smoothly fitted in respective accesses of the mixing chamber. Such blended transitions considerably help reducing backpressure.
For a simplified introduction of the reducing agent into the mixing chamber and a good mixing performance it has further turned out to be advantageous if the inlet pipe and the outlet pipe form an angle of about 90°.
The features and feature combinations mentioned above in the description as well as the features and feature combinations mentioned below in the description of figures and/or shown in the figures alone are usable not only in the respectively specified combination, but also in other combinations or alone without departing from the scope of the invention.